The present invention relates to method of operating a magnetically driven vibrating conveyor having a dual mass vibratory system under different conveying loads with a constant excitation frequency and to an apparatus for implementing the method. Such a method is disclosed in the in-house publication entitled, "Grundbegriffe der Schwingfordertechnik"[Basic Concepts of Vibratory Conveying], by AEG-Telefunken, Vibration and Welding Division.
The following discussion refers to FIGS. 1 and 2 of the present drawings, and it is assumed that the vibrating conveyor has a magnetic vibrator as its drive.
The vibrating conveyor has a natural, that is, resonant frequency f.sub.e which shifts toward lower values with increasing masses in the dual mass vibratory system. In a known manner, the resonance curve is a function of the excitation frequency f.sub.a, i.e. it is flatter, the greater the attenuation. FIG. 2 shows vibration amplitude as a function of the quotient formed from the excitation frequency and the natural frequency are nearly the same. Operation of the vibrating conveyor in the resonance range where the excitation frequency of the magnetic drive and natural frequency of the conveyor is more critical and labile with respect to the vibration amplitude, the less attenuation there is. On the other hand, it is desirable to have as little attenuation as possible in order to keep power losses low. The operating point is therefore customarily placed on the left or right branch of the resonance curve. Operation is more stable the farther the operating point is removed from the point of resonance where the excitation and natural frequencies are equal. Consideration is first given to the case of supercritical operation, that is, the case where the natural frequency is lower than the excitation frequency (f.sub.a /f.sub.e &gt;1.0), with it being assumed that the excitation frequency is a constant value. By increasing the conveying material load, attenuation is increased and the vibration amplitude is reduced by the path E-F. This increase in attenuation is explained by the fact that, for a conveying material that is increasingly granular, the sum of the friction between individual bodies becomes higher, resulting in an increase in attenuation. The increased conveying material load simultaneously couples in a larger mass so that the natural frequency of the vibrating conveyor drops (path F-G). This not only increases the distance from the point of resonance but also causes the amplitude of the vibrations to be reduced.
In subcritical operation, the natural frequency is higher than the excitation frequency so that f.sub.a /f.sub.e &lt;1.0. In contrast to supercritical operation, the coupling in of a larger mass results in an increase in amplitude (curve A-B). Here, again, however, the vibration amplitude decreases with increasing attenuation (curve B-C). Attenuation and coupling in of masses thus counteract one another, resulting in stabilization of operation. Since in the case of resonance the vibrations are unstable, the system is set so that, under consideration of all influences from the load, the distance from the point of resonance is so great that stable operation results. Operation at resonance is not possible without additional measures. In order to keep the influence of coupling in and attenuation by the conveyed material as low as possible, it is possible to select the masses of the operating side of the vibrating conveyor, and particularly those of the free side as large as possible. This results in good vibration stability even near resonance. However, such a design requires that the magnetic vibrator have a relatively large mass which would be undesirable from a point of view of maneuverability and also for reasons of costs. The optimum ratio of operating mass to free mass lies at about 3 : 1 to 4 : 1. In this case, the vibratory system is customarily adjusted so that, under maximum influence from the load, a distance from the point of resonance of more than about 5% results.
The control of an electromagnetically excited dual mass vibratory system is disclosed in German Auslegesschrifft AS 1,111,270. The vibration amplitude corresponding to the end position of a mass in a vibratory drive can be adjusted by connecting transformers, a choke or resistors to its input. In order to regulate a given vibration amplitude to the desired value, a magnetic amplifier is connected to the input of the vibratory drive and is regulated automatically as a function of the difference between the greatest and the smallest distance between the operating and free side masses. In this connection, it is also known to perform resonance vibration control (e.g. see Chemie-Technik [Chemical Engineering], Volume 14 (1985), No. 12, pages 45 and 46) to effect amplitude vibration control. For this purpose, a frequency feedback is provided from the vibrating conveyor to the control circuit which thus synchronizes itself to the natural resonance of the conveyor system. In order to prevent the conveying amplitude from overshooting, a further amplitude control circuit is required to control the excitation energy. Although this control ensures an accurately proportional behavior of the vibration amplitude relative to the set value, independently of all extraneous influences, it is extremely expensive since no fixed relationship exists between the excitation frequency and the mains frequency. Frequency control is necessary.
In summary, it can thus be stated that for optimum conveying operations, a vibrating conveyor, its adjustment and its vibratory drive must meet the following conditions:
In order to realize good efficiency, work should take place as close to resonance as possible since, except for attenuation losses, the full excitation energy is transferred to the vibrating conveyor. Relative to the working mass, the free mass should be as small as possible. Since overshooting of the operating amplitude could have the result that core and armature of the vibratory drive hit one another in the air gap and might possibly destroy one another, such undesirable overshooting must be prevented.